Field of the Invention
The present invention relates to ophthalmic lenses, and more particularly, to contact lenses designed to slow, retard, or prevent myopia progression. The ophthalmic lenses of the present invention comprise multifocal power profiles that provide foveal vision correction, an increased depth of focus and an optimized retinal image at a range of accommodative distances that makes the degradation of retinal image quality less sensitive to blur during near work activities, thereby preventing and/or slowing myopia progression.
Discussion of the Related Art
Common conditions which lead to reduced visual acuity include myopia and hyperopia, for which corrective lenses in the form of spectacles, or rigid or soft contact lenses, are prescribed. The conditions are generally described as the imbalance between the length of the eye and the focus of the optical elements of the eye. Myopic eyes focus in front of the retinal plane and hyperopic eyes focus behind the retinal plane. Myopia typically develops because the axial length of the eye grows to be longer than the focal length of the optical components of the eye, that is, the eye grows too long. Hyperopia typically develops because the axial length of the eye is too short compared with the focal length of the optical components of the eye, that is, the eye does not grow long enough.
Myopia has a high prevalence rate in many regions of the world. Of greatest concern with this condition is its possible progression to high myopia, for example greater than five (5) or six (6) diopters, which dramatically affects one's ability to function without optical aids. High myopia is also associated with an increased risk of retinal disease, cataracts, and glaucoma.
Corrective lenses are used to alter the gross focus of the eye to render a clearer image at the retinal plane, by shifting the focus from in front of the plane to correct myopia, or from behind the plane to correct hyperopia, respectively. However, the corrective approach to the conditions does not address the cause of the condition, but is merely prosthetic or intended to address symptoms. More importantly, correcting the myopic defocus error of the eye does not slow or retard myopia progression.
Most eyes do not have simple myopia or hyperopia, but have myopic astigmatism or hyperopic astigmatism. Astigmatic errors of focus cause the image of a point source of light to form as two mutually perpendicular lines at different focal distances. In the following discussion, the terms myopia and hyperopia are used to include simple myopia or myopic astigmatism and hyperopia and hyperopic astigmatism respectively.
Emmetropia describes the state of clear vision where an object at infinity is in relatively sharp focus with the crystalline lens relaxed. In normal or emmetropic adult eyes, light from both distant and close objects and passing though the central or paraxial region of the aperture or pupil is focused by the crystalline lens inside the eye close to the retinal plane where the inverted image is sensed. It is observed, however, that most normal eyes exhibit a positive longitudinal spherical aberration, generally in the region of about +0.50 Diopters (D) for a 5.0 mm aperture, meaning that rays passing through the aperture or pupil at its periphery are focused +0.50 D in front of the retinal plane when the eye is focused to infinity. As used herein the measure D is the dioptric power, defined as the reciprocal of the focal distance of a lens or optical system, in meters.
The spherical aberration of the normal eye is not constant. For example, accommodation (the change in optical power of the eye derived primarily though changes to the crystalline lens) causes the spherical aberration to change from positive to negative.
As noted, myopia typically occurs due to excessive axial growth or elongation of the eye. It is now generally accepted, primarily from animal research, that axial eye growth can be influenced by the quality and focus of the retinal image. Experiments performed on a range of different animal species, utilizing a number of different experimental paradigms, have illustrated that altering retinal image quality can lead to consistent and predictable changes in eye growth.
Furthermore, defocusing the retinal image in both chick and primate animal models, through positive lenses (myopic defocus) or negative lenses (hyperopic defocus), is known to lead to predictable (in terms of both direction and magnitude) changes in eye growth, consistent with the eyes growing to compensate for the imposed defocus. The changes in eye length associated with optical blur have been shown to be modulated by changes in scleral growth. Blur with positive lenses, which leads to myopic blur and a decrease in scleral growth rate, results in development of hyperopic refractive errors. Blur with negative lenses, which leads to hyperopic blur and an increase in scleral growth rate, results in the development of myopic refractive errors. These eye growth changes in response to retinal image defocus have been demonstrated to be largely mediated through local retinal mechanisms, as eye length changes still occur when the optic nerve is damaged, and imposing defocus on local retinal regions has been shown to result in altered eye growth localized to that specific retinal region.
In humans there is both indirect and direct evidence that supports the notion that retinal image quality can influence eye growth. A variety of different ocular conditions, all of which lead to a disruption in form vision, such as ptosis, congenital cataract, corneal opacity, vitreous hemorrhage and other ocular diseases, have been found to be associated with abnormal eye growth in young humans, which suggests that relatively large alterations in retinal image quality do influence eye growth in human subjects. The influence of more subtle retinal image changes on eye growth in humans has also been hypothesized based on optical errors in the human focusing system during near work that may provide a stimulus for eye growth and myopia development in humans.
One of the risk factors for myopia development is near work. Due to accommodative lag or negative spherical aberration associated with accommodation during such near work, the eye may experience hyperopic blur, which stimulates myopia progression as discussed above.
Moreover, the accommodation system is an active adaptive optical system; it constantly reacts to near-objects, as well as optical designs. Even with previously known optical designs placed in front of the eye, when the eye accommodates interactively with the lens+eye system to near-objects, continuous hyperopic defocus may still be present leading to myopia progression. Therefore, one way to slow the rate of myopia progression is to design optics that reduces the impact of hyperopic blur on retinal image quality. With such designs, for each diopter of hyperopic defocus the retinal image quality is less degraded. In another sense, the retina is therefore relatively desensitized to hyperopic defocus. In particular, depth of focus (DOF) and image quality (IQ) sensitivity may be used to quantify the susceptibility of the eye to myopia progression as a result of hyperopic defocus at the retina. An ophthalmic lens design with a larger depth of focus and low image quality sensitivity will make the degradation of retinal image quality less sensitive to hyperopic defocus, hence slowing down the rate of myopia progression.
In object space, the distance between the nearest and farthest objects of a scene that appear acceptably sharp is called depth of field. In image space, it is called depth of focus (DOF). With a conventional single vision optical design, a lens has a single focal point, with image sharpness decreasing drastically on each side of the focal point. With an optical design with extended DOF, although it may have a single nominal focal point the decrease in image sharpness is gradual on each side of the focused distance, so that within the DOF, the reduced sharpness is imperceptible under normal viewing conditions.
Image quality (IQ) sensitivity can be defined as the slope of the retinal IQ-defocus curve at an accommodative demand of 1 to 5 diopters. It indicates how image quality changes with defocus. The larger the value of IQ sensitivity, the more sensitive image quality is to defocus error during accommodation.